This application claims priority of French Patent Application No. 01 00471, filed on Jan. 15, 2001.
The invention relates to installations for the circulation of part-carrying pallets.
It more particularly concerns an installation for the circulation of part-carrying pallets of a generally rectangular shape which are displaced in elementary modules under the frictional action of at least one drive run, displacement of the pallets being effected by way of rolling means fitted on the pallet and rolling on the respective bottoms of the modules.
Installations of that type, also referred to as xe2x80x98flexible workshopsxe2x80x99, are already known, in which the modules can accommodate working stations and/or offer pure circulation functions.
Usually, each of the modules comprises at least one drive run such as for example an endless belt which provides for the displacement of a pallet in the interior of the module and from one module to another.
Those different modules define circulation paths of a selected shape, being generally straight or curved, possibly with a branch path.
Such installations make it possible to displace a pallet between different working stations in order to carry out a succession of operations, for example assembly and/or machining operations, on the parts which are carried by the pallets. Those installations thus make it possible to provide for the production of parts by a succession of operations which are implemented manually and/or automatically at the working stations.
A known installation of that type is described in French patent No 85 16095, published under the No 2 589 134. In that known installation, the pallets are driven by the co-operation of a drive run with a friction member forming part of the pallet and caused to bear under an elastic pressure against the run.
Such installations have been perfectly satisfactory hitherto, having regard to the relatively low speeds and levels of acceleration with which the pallets are involved.
However, with progress in robotics, the pallets are being displaced at ever increasing speeds, while also being subjected to ever increasing levels of acceleration.
Moreover, having regard to the ever increasing rates required for the production of parts, it is imperative that the pallets can be immobilised in a precise position at the location of a working station, more specifically with a very short time interval, in order then to be freed and continue their displacement towards other working stations.
It is necessary for the resumption of movement of the pallet under the effect of the drive run, at the end of immobilisation thereof, to be effected under optimum conditions in spite of the frictional factors involved.
It is imperative in particular that such resumption of movement does not give rise to acceleration effects, shocks nor vibrations, which are excessive and which would be likely to interfere with the movement of the pallet and/or cause displacement of the part or parts that the pallet is carrying.
Another problem which arises is that of ensuring entrainment of the pallets by frictional engagement with the drive run, immobilisation of the pallet while the drive run continues its movement, then resumption of the movement of the pallet under optimum operating conditions, all this in spite of the high speed of movement of the drive run.
The object of the present invention is precisely to provide an installation of that type, which makes it possible to afford circulation of the pallets under optimum operating and safety conditions, even at high speeds (typically of the order of several meters per second), as required by the current progress in robotics.
Another object of the invention is to provide such an installation in which the problems raised by circulation of the pallets at high speed, then immobilisation thereof and then resumption of the circulation thereof, are resolved in a simple and effective manner.
Yet another object of the invention is to afford such an installation in which immobilisation or indexing of a pallet which is in motion can be achieved without difficulty, even in the situation where the pallet is carrying a heavy load and is circulating at high speed.
A further object of the invention is to provide such an installation in which precise immobilisation of a pallet can be achieved by simple means, for the purposes of treating or processing the part or parts that the pallet is carrying.
The invention is applied to installations in which the modules provide for displacement on a circulation path of a selected shape, with or without maintenance of the spatial orientation of the pallet.
A further object of the invention is to provide a pallet suitable for being part of such an installation.
For that purpose the invention proposes an installation of the type defined hereinbefore, in which the pallet comprises a body below which are mounted idler wheels disposed horizontally to provide for guiding and/or driving of the pallet, in which each module comprises at least one guide rail defining a circulation path of selected shape and made of an elastically deformable material and in which during displacement of the pallet on the module the guide rail exercises an elastic return force against at least one idler wheel of the pallet in such a way as to urge at least one drive portion of the pallet into elastic contact against the drive run of the module.
The guide rail, by virtue of its elastic properties, exercises a spring function which urges the drive portion of the pallet against the drive run. That spring function of the guide rail can be reinforced or quite simply replaced by the addition of one or more mechanical flaps, which for example are spring-mounted, on the drive portion of the pallet to improve traction.
Thus, displacement of the pallet in a module or from one module to another is effected by frictional engagement of a drive portion of the pallet against a drive run, under the effect of an elastic return force exerted by a guide rail against at least one idler wheel of the pallet.
As will be seen hereinafter, those idler wheels which can also be referred to as rolling bearings contribute to guiding and driving of the pallet during the displacement thereof.
An important feature lies in the fact that the elastic return force which contributes to frictional engagement of the pallet with the drive run is ensured by the guide rail which accordingly ensures a combined guidance and elastic return function.
That therefore avoids having recourse to a specific friction member forming part of the pallet and caused to bear under elastic pressure against the run, as was the case in the installation in the above-mentioned French patent.
The installation of the invention may comprise different types of modules.
In particular, this may involve a module which defines a straight circulation path and which comprises a straight drive run, as well as a straight guide rail, which are in mutually parallel relationship, the drive portion of the pallet being formed by a side of the pallet while the idler wheel or wheels of the pallet which bear against the straight guide rail is or are disposed in the proximity of an opposite side of the pallet.
Thus, in that case, the drive portion of the pallet is formed by a side thereof.
The module may also define a curved circulation path and may comprise a curved drive run and a curved guide rail, which are disposed at a constant spacing, the drive portion of the pallet being formed by a first region of at least one idler wheel of the pallet which has a second region disposed in opposite relationship to the first region and bearing against the guide rail in the form of a circular arc.
In that case, drive and guidance for the pallet are afforded by at least one idler wheel which is urged into a condition of bearing against the drive run by way of the guide rail.
The module may also define a straight main circulation path having a straight main drive run, a straight secondary circulation path having a straight secondary drive run, with the main paths and secondary paths forming a right-angled branching. In that case the module is fitted with a diversion or direction-changing roller (advantageously being a drive roller), with a vertical axis, which is displaceable vertically between a low position in which it disappears into the bottom of the module for the pallet to be displaced along the main path, and a high position in which it projects above the bottom of the module in order to engage into a curved guide groove provided in one of the corners of the pallet in such a way that the pallet leaves the main circulation path, coming out of contact with the main drive run, to engage into the secondary circulation path and come into contact with the secondary drive run, without a modification in the spatial orientation of the pallet.
The diversion roller may also be accompanied by another member for facilitating movement in a straight line, for example another roller which can be raised in alternate relationship with the diversion roller.
The invention also provides indexing means for immobilising the pallet at a selected location on the circulation path, such means comprising two thrust members capable of being actuated by way of a sensor to apply a lateral force to two idler wheels of the pallet, on the opposite side to the drive run, cause disengagement of the drive portion of the pallet from the drive run, preferably without altering the linear speed thereof, and thus arrest the pallet.
Those indexing means may also comprise a retractable damper capable of being actuated with the thrust members to co-operate with an abutment of the pallet.
In another aspect the invention concerns a pellet capable of being used in an installation as defined hereinbefore.